1. Field of the Invention
The present invention relates to a process for preparing extruded polymeric foam.
2. Description of Related Art
Extrusion processes for preparing polymeric foam include three basic steps: (1) providing a foamable polymer composition at an initial pressure and temperature in an extruder; (2) expelling the foamable polymer composition into a zone of lower pressure; and (3) allowing the foamable polymer composition to expand into polymeric foam. The third step can occur with or without constraint on the expanding foamable polymer composition. Such a general process works well for many types of extruded polymeric foams but offers little control over foam expansion. Consequently, inhomogeneous foam properties develop when preparing extruded polymeric foams requiring large thickness expansion ratios. Important inhomogeneities in density and compressive strength often appear along the thickness dimension of extruded polymeric foam when prepared by a typical extrusion process. These inhomogeneities become apparent when the expanding foam undergoes an expansion ratio of 30:1 or more in its thickness dimension. Nonetheless, expansion ratios of 30:1 or more are desirable for preparing low density and/or thick polymeric foam such as foam having a thickness of 100 millimeters or more.
It is desirable and would advance the art of extruded polymeric foam to identify a process for directly preparing polymeric foam having a thickness expansion ratio in excess of 30:1 while also having an essentially uniform density and compressive strength along the thickness dimension.